As the performance demands on modern aircraft increase, there becomes an ever-increasing need for additional cooling of the engine and airframe. Where the aircraft is powered by a gas turbine engine, this cooling function can be achieved by bleeding air from the compressor section of the engine.
However, as aircraft speeds increase, the cooling capacity of this bleed air decreases due to the increase in its stagnation temperature. This combined with the increased cooling requirements of the aircraft at higher speeds, makes the use of bleed air for secondary cooling both less efficient and less practical.
Aviation fuels, in addition to their primary use as a propellant, are often used as a coolant for the engine lubricants and environmental control and avionics systems in aircraft. This increases the thermal loads experienced by aviation fuels, with these thermal loads being anticipated to rise with the requirements of advanced aircraft.
As a result, the thermal stability of aviation fuels is becoming increasingly important to the design of high performance aircraft systems.
It is well known that the exposure of hydrocarbon based fuels to elevated temperatures may result in oxidative degradation (“auto-oxidation”) and producing insoluble products in the bulk liquid as well as forming deposits on fuel washed surfaces. Such insoluble products and deposits can result in reduction in heat exchanger efficiencies as well as causing obstructions in flow components such as fuel injectors, pipes, filters and valves.
One approach to reducing the vulnerability of a hydrocarbon fuel to auto-oxidation and thereby to improve its thermal stability is the addition of additive compounds to the fuel. Fuel additives, while effective, increase the cost of the fuel and cause logistical problems insofar as the fuel additive must be distributed and blended with the fuel before use.
A major factor which adversely affects a hydrocarbon fuel's thermal stability is the quantity of dissolved oxygen in the fuel. Due to the fuel's affinity for oxygen, the exposure of the fuel to the atmosphere can result in the absorption of oxygen by the fuel.
A known technique for removing dissolved oxygen from, or de-oxygenating, a fuel involves a membrane separator, or molecular filter. This consists of an ultra-thin membrane layered with a micro-porous polymer support that provides strength.
The membrane separation technique while effective requires dedicated hardware and control mechanisms which add to the complexity and weight of the aircraft.